1. Field of the Invention
The present invention relates to an ultra-sensitive metal oxide gas sensor and a fabrication method thereof, and more particularly to a fabrication method of an ultra-sensitive metal oxide gas sensor, in which a fine structure of a metal oxide is a network structure of nanofibers composed of nanograins or nanorods composed of nanograins.
2. Description of the Background Art
When ZnO, SnO2, WO3 and TiO2, which are metal oxide semiconductors, contacts special gas elements such as H2, CO, O2, NOx, CO2, DMMP, CH4, NH3, Alcohol and humidity, an electrical resistivity is changed by gas adsorption and oxidization/reduction occurring on the surface of the oxide.
Characteristics of a sensor fabricated by using the metal oxide semiconductor are considerably influenced by gas diffusivity and gas surface reaction. Accordingly, efforts have been made to increase an enhanced surface activity and a surface to volume ratio.
For example, studies of humidity and ammonia sensor using a ZnO nano wire structure have been made [Y. S, Zhang, Physica B-Condense Matter. Vol. 368, 94-99, 2005 or X. H. Wang, Appl. Phy. Lett. Vol. 252, 2504-2411, 2005]. Also, characteristics of a SnO2 [Zhang, D. Nano Lett. 4, 1919, 2004] and In2O3 [Kolmakov, A. Nano Lett. 5, 667, 2005] sensor using nanowire have been studied [Sens. Actuators B, 108, 29, 2005].
A sensor using a single nanowire can obtain high sensitivity but has a problem in that it is difficult to fabricate a device having high reproducibility due to noise caused by unstability of contact resistance.
Thus, sensors fabricated using networks of nanofibers may offer high reproducibility and better electrical stability as compared to that of sensors using individual nanowire and nanofiber. Electrospinning is one of the most simple and versatile approaches offering the ability to produce multiple nanofiber networks.
The fiber prepared by the electrospinning has a diameter of a few tens nm to a few μm depending on preparation conditions. Therefore, a surface area per unit volume of electrospun nano fibrous mats is larger than that of a continuous film by a few hundreds times (two orders of magnitude). When the fiber is used as the sensor material, the sensor is expected to have high sensitivity and rapid response. In this respect, studies for use in a chemical sensor, an optical sensor, and a bio sensor have been made actively.
Electrospun nanofibrous mats having a diameter of several hundreds of micrometers, fabricated by electrospinning have been studied by various research groups [D. Li and Y Xia, Nano Lett. 3 (2003), 555]. Such electrospun nanofibrous mats have high gas diffusivity due to an excellent porous structure of an open pore structure. However, since the size of the nanofiber ranges a few hundreds nm, a specific surface area is very low, generally below 20 to 30 m2/g. A sensor consisting of such materials enables fabrication of a large sized nanofiber, can obtain a network of an ultra-fine nanofiber at a low cost, and has sensing sensitivity more improved than that of a metal oxide nanowire obtained by thermal vapor deposition but still has deficient sensitivity.
There are increasing demands for a high sensitivity sensor of a thin layer of a fiber having an increased specific surface area of a metal oxide along with a porous structure having excellent gas diffusivity to attain a fast response time and high sensitivity sensing.
In addition, since adhesion between a metal oxide fiber and a sensing substrate is closely related to electrical contact, excellent adhesion is required to minimize noise. If a composite nanofiber including an electrospun metal oxide precursor is thermally treated on a metal or a ceramic substrate at a high temperature, peeling-off of nanofibers with respect to the substrate is observed.